Splitting a bank transaction

I have a transaction with a vendor that is a return of goods that needs to be split between 2 customers.  in accounting view, when i select the transaction and then split, i am only able to split it between categories and class.

In reviewing past transactions, i have been able to split them but I do not see how.  It wont allow me to do a negative expense to create a match.  A vendor credit does not show up as a possible match in the banking transaction pages.

What am I missing to split a transaction between 2 customers for a return of goods?